The present invention relates to electrical resistance corrosion and plating sensors, and more particularly concerns sensors that are of increased sensitivity for monitoring corrosive tendencies or plating processes.
A common method of continuous measurement of corrosion characteristics employs resistance measurement of a metallic, corrodible or erodible test element to indicate, by change of resistance, the amount of metal that has been lost by corrosion or erosion over a period of time. A widely used sensor for this measurement is known under the trademark Corrosometer manufactured by Rohrback Corporation, assignee of this application. One such sensor employs a tubular metallic test element loop or wire, part of which operates as a reference element made of the same material as the test element. The reference element is protected from the environment while the test element is exposed to the environment. Alternating current is passed through the elements and electrical resistance of each is measured while or after the sensor has been immersed in an environment of which corrosive tendencies are to be monitored. Because resistance varies with the amount of metal in the test element, measurement of test element resistance provides an indication of metal loss and, therefore, of corrosion. However, because resistance of the metal also changes with temperature, a reference element is provided, made of the same material as the test element, and having the same temperature resistance characteristics. Thus, changes in resistance of the test element that are due to long term, relatively slow temperature variation may be eliminated by comparison of resistances of the test and reference elements. Other sensors may include a cylindrical metallic test element carrying an inner reference element made of the same material as the test element with the interior of the test element filled with a thermally conductive, electrically nonconductive compound, thereby providing physical support for the preferably very thin wall of the test element.
A major shortcoming of presently available electrical resistance corrosion sensors is lack of desired sensitivity to low corrosion rates (in the order of less than about 2 mils per year) due to physical constraints imposed by pressures and flow conditions in a process environment that is being monitored. For structural reasons the wall thickness of tubular loop test elements or straight cylindrical test elements is usually required to be 4 mils or greater. Similarly, the diameter of a wire loop must be at least 20 mils for adequate physical structure and strength necessary to withstand the process pressures and flow conditions.
Sensitivity of a corrosion sensor is directly related to its initial resistance and, therefore, to the initial thickness of the test material. Sensitivity is inversely related to the test material thickness. To provide increased sensitivity of such corrosion sensors it has been suggested to employ a relatively thin coating on a relatively thick ceramic or other substrate that is not electrically conductive. However, there are significant problems involved in the accurate coating of different ones of the desired test materials upon the nonconductive substrate. Such manufacturing problems and costs have heretofore made such probes practically unfeasible. Thus, corrosion sensors of the prior art are of a limited sensitivity.
In the electroplating of various objects, monitoring of the plating process has been carried out by periodically removing a test coupon from the bath and measuring its plating thickness. Precision measurement of plating thickness as the electroplating process proceeds has not been available. Electrical resistance measurements, for example, those made by measuring the continually decreasing resistance of a plated test coupon during the progress of an electrolytic plating process, have not been possible. The electrical current flowing in the plating tank causes sufficient interference to the measuring circuitry that measurement of resistance is impossible. Moreover, such sensors, when used for monitoring corrosive tendencies of a fluid environment, are generally not renewable and must be discarded after a finite period of use. These sensors are relatively expensive, at least in part due to manufacturing procedures which are necessarily intricate and highly precise, and therefore, renewability is desirable.
Accordingly, it is an object of the present invention to enable sensing of plating processes or corrosive tendencies in a manner which eliminates or minimizes above-mentioned problems.